Safety interlock circuit



Feb. 25, 1958 D. R. HAUSTEN 2,82 ,0 0

SAFETY INTERLOCK CIRCUIT Filed Feb. 8, 1955 .INVENTOR. DAVID R. HA usTEN AT'roR'lvEy Unite States Patent SAFETY INTERLOCK CIRCUIT David R. Hausten, Wheaten, Ill., assignor to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application February 8, 1955, Serial No. 486,952

Claims. (Cl. 340-282) This invention relates in general to a method of controlling power application to a transmitter, or any other electrical device, from a remote location. In particular this invention relates to a safety interlock circuit which allows power to be applied to the device by operation of one switch but requires operation of two switches to remove power.

In certain cases where transmitters or other electrical devices are used for remote control of other equipment it is desirable that the controlling equipment not be turned off accidentally.

The object of this invention is to provide a method of power-application control which prevents power from being accidentally turned off by operating personnel.

With this invention power is applied by operation of a main control switch. However, to turn power off the main control switch and a supplementary control switch must be operated in sequence. A warning indicator, which comes on when the main control switch has been placed in the off position, indicates when operation of the supplementary control switch will remove power from the equipment.

Further objects, features and advantages of the invention will become apparent from the following description and claims when read in view of the drawing, in which the figure is a circuit diagram of the invention.

Referring to the figure, the power control line from a transmitter or other electrical device is connected to contact 11 of main control switch S1 and by lead 12 to contact 13 on relay K1. Completion of either of these circuits will ground the power control line and cause power to be applied to the equipment. Main control switch S1 is a standard double-pole, double-throw switch. Contact 14 on the switch is open circuited. Contact 15 on the switch is connected to ground and moveable between contacts 11 and 14. Contact 16 is moveable between contacts 17 and 18 and is connected to ground through warning indicator I. Relay K1 is a sensitive double-pole, double-throw switch relay with 8000 ohms resistance.

A moveable, grounded contact 19 on relay K1 engages contact 20 when the relay is not energized and contact 13 when the relay is energized. A second moveable, grounded contact 21 on relay K1 engages contact 22 when the relay is not energized and contact 23 when the relay is energized. Contact 22 is open circuited. Contact 23 is connected with lead 24 to contact 25 on supplementary control switch S2. This switch is a normally closed switch which opens when operated and closes when released.

Relay K1 is connected with lead 26 to contact 27 on switch S2, and with lead 28 to contact 17 on switch S1. The other side of relay K1 is connected with lead 29 to resistor R1. Resistor R1 is connected to +90 volts with lead 30. Resistor R2 is connected to +28 volts with lead 31. The minus side of the 28 volt source is connected to ground. Resistor R2 is connected with lead 32 to contact 18 on switch S1. Contact 18 on switch S1 is also connected with lead 33 to contact 20 on relay K1.

When the main control switch S1 is placed in the on 2,825,050 Patented Feb. 25, 1958 ICC position the power control line 10 is grounded through switch contacts 11 and 15. This causes power to be applied. At the same time power is turned on with S1, volts is applied to relay K1 through resistor R1, contacts 17 and 16 on switch S1 and warning indicator I to ground. The resistance of the warning indicator I is so low compared to the resistance of resistor R1 and relay K1 that it does not come on. However, the relay K1 is energized and relay contacts 13-19 and 2321 are closed. The closing of relay K1 contacts 13 and 19 causes the power control line to be grounded in parallel with the ground circuit through switch S1 contacts 11 and 15. Closing of relay K1 contacts 23 and 21 completes a holding circuit [for relay K1 through supplementary control switch S2.

Power can be turned off only by operating switch S1 and switch S2 in that sequence. If switch S2 is operated after power has been applied by action of switch S1, the power control line remains grounded through contacts 11 and 15 on switch S1, and relay K1 remains grounded through contacts 17 and 16 on switch S1. Since the relay K1 is energized, the power control line also remains grounded through relay contacts 13 and 19. Switch S2 will return to normally closed position when released. If switch S1 is moved to oil position, the power control line ground circuit through switch S1 contacts 11 and 15 is broken. However, relay K1 is held closed because of the holding circuit through switch S2 and relay contacts 23 and 21. When switch S1 is in the off" position and relay K1 is energized +28 volts is applied to the warning indicator I through resistor R2 and switch S1 contacts 18 and 16. This causes the indicator to come on, indicating that operation of switch S2 will turn ofi power to the equipment.

When switch S2 is opened after switch S1 has been moved to the ofi position, the holding circuit for relay K1 through switch S2 and relay contacts 23 and 21 is broken. This causes relay K1 contacts 13 and 19 to open removing the last ground from the power control line. When relay K1 is de-energized, relay K1 contacts 19 and 20 are closed and +28 volts is shorted through resistor R2 and the contacts to ground. This removes the voltage from the warning indicator I and causes the indicator to go out, thus indicating that the power-01f sequence has been completed.

This invention assures that the power control line will not be accidentally ungrounded by requiring switches S1 and S2 to be operated in that sequence, and by supplying a warning indicator which indicates when operation of S2 will complete the power-oft" sequence.

I claim:

1. A safety interlock circuit comprising, a first control switch which is double-pole, double-throw, a second control switch which is single-throw, single-pole, a power control line connected to a first contact on the first control switch, a first movable contact on the first switch connected to ground and movable between first and second stationary contacts, said second contact open-circuited, a relay with one side connected to a power supply, the other side of the relay connected to the third stationary contact of the first switch, an indicator connected in series to ground with the second movable contact of the first control switch, said second movable contact movable between the third and fourth stationary contacts of the first switch, a second voltage source connected between ground and the fourth stationary contact of the first switch. a pair of relay switches actuated by said relay and with their movable contacts connected to ground, a first stationary contact of the first relay switch connected to a first stationary contact of the first control switch, the second stationary contact of the first relay switch connected to the fourth stationary contact of the first control switch, the

first stationary contact of the second relay switch connected to one side of the second control switch, the other side of the second control switch connected to the relay.

2. A safety interlock circuit comprising, a first control switch which is double-pole, double-throw, a second control switch which is single-throw, single-pole, a power control line connected to a first contact on the first control switch, a first movable contact on the first switch connected to ground and movable between first and second stationary contacts, said second contact open-circuited, a relay with one side connected to a power supply, the other side of the relay connected to the third stationary contact of the first switch, a second movable contact of the first switch connected to ground, said second movable contact movable between the third and fourth stationary contacts of the first switch, a second voltage source connected between ground and the fourth stationary contact of the first switch, a pair of relay switches actuated by said relay and with their movable contacts connected to ground, a first stationary contact of the first relay switch connected to the first stationary contact of the first control switch, the second stationary contact of the first relay switch connected to the fourth stationary contact of the first control switch, the first stationary contact of the second relay switch connected to one side of the second control switch, the other side of the second control switch connected to the relay.

3. A safety interlock circuit comprising, a first control switch which is double-pole, double-throw, a second control switch which is single-throw, single-pole, a power control line connected to a first contact on the first control switch, a first movable contact on the first switch connected to ground and movable between first and second stationary contacts, said second contact open-circuited, a relay with one side connected to a first resistor, the other side of the first resistor connected to a power supply, the

' other side of the relay connected to the third stationary contact of the first switch, an indicator connected in series to ground with the second movable contact of the first control switch, said second movable contact movable between the third and fourth stationary contacts of the first switch, a second voltage source, a second resistor connected between the fourth stationary contact of the first switch and the second voltage source, a pair of relay switches actuated by said relay and with their movable contacts connected to ground, a first stationary contact of the first relay switch connected to the first stationary contact of the first control switch, the second stationary contact of the first relay switch connected to the fourth stationary contact of the first control switch, the first stationary contact of the second relay switch connected to one side of the second control switch, the other side of the second control switch connected to the relay.

4. A safety interlock circuit comprising, a first control switch which is double-pole, double-throw, a second control switch which is single-throw, single-pole, a power control line connected to a first contact on the first control switch, a first movable contact on the first switch connected to ground and movable between first and second stationary contacts, said second contact open-circuited, a relay with one side connected to a resistor, the other side of the resistor connected to a first power supply, the other side of the relay connected to the third stationary contact of the first switch, the second movable contact of the first control switch connected to ground, said second movable contact movable between the third and fourth stationary contacts of the first switch, a second voltage source connected between ground and the fourth stationary contact of the first switch, a pair of relay switches actuated by said relay and with their movable contacts connected to ground, first stationary contact of the first relay switch connected to the first stationary contact of the firs control switch, the second stationary contact of the first relay switch connected to the-fourth stationary contact of the first control switch, the first stationary contact of the second relay switch connected to one side of the second control switch, the other side of the second control switch connected to the relay.

5. A safety interlock circuit comprising, a first control switch which is double-pole, double-throw, a second control switch which is single-throw, single-pole, a power control line connected to a first contact on the first control switch, a first movable contact on the first switch connected to ground and movable between first and second stationary contacts, said second contact open-circuited, a relay with one side connected to a power supply, the other side of the relay connected to the third stationary contact of the first switch, an indicator connected in series to ground with the second movable contact of the first control switch, said second movable contact movable between the third and fourth stationary contacts of the first switch, a second voltage source, a resistor connected between the fourth stationary contact of the first switch, and the second voltage source, a pair of relay switches actuated by said relay and with their movable contacts connected to ground, a first stationary contact of the first relay switch connected to the first stationary contact of the first control switch, the second stationary contact of the first relay switch connected to the fourth stationary contact of the first control switch, the first stationary contact of he second relay switch connected to one side of the second control switch, the other side of the second control switch connected to the relay.

References Cited in the file of this patent UNITED STATES PATENTS 

